The heart of a computer is a magnetic disk drive that includes a magnetic disk, a slider where a magnetic head assembly including write and read heads is mounted, a suspension arm, and an actuator arm. The slider is biased by the suspension arm toward the surface of the magnetic disk. When the magnetic disk rotates, the rotating magnetic disk moves air adjacent an air bearing surface (ABS) of the slider, causing the slider to fly on the air bearing. When the slider flies on the air bearing, the actuator arm swings the suspension arm to place the magnetic head assembly over selected circular tracks on the rotating magnetic disk, where signal fields are written and read by the write and read heads, respectively. The write and read heads are connected to processing circuitry that operates according to a computer program to implement write and read functions.
An exemplary high performance read head employs a magnetoresistive read element for sensing the signal fields from the rotating magnetic disk. A magnetoresistive sensor detects magnetic field signals through the resistance changes of a read element as a function of the strength and direction of magnetic flux being sensed by the read element. In the last decade, such read elements have been predominantly in the form of giant magnetoresistive sensors (GMR). More recently, Tunnel Junction sensors have been investigated for use as magnetic read elements.
A GMR element also referred to in the art as a spin valve magnetoresistive element includes a pair of ferromagnetic layers separated by a non-magnetic electrically conductive spacer layer. The magnetization of one of the layers is fixed or “pinned” while the other is biased but free to rotate in response to a magnetic field. Due to the spin dependent scattering of electrons through the spacer layer, current flow through the spacer layer depends upon the relative orientation of the magnetization of the free and pinned magnetic layers. Current flow is at a maximum when the free and pinned layers are magnetized parallel to one another, whereas current flow is at a minimum when the magnetizations are anti-parallel. The free and pinned layers are generally set at 90 degrees to one another in the absence of a magnetic field in order to generate the most linear resistance change in response to a magnetic field signal. Most GMR sensors in use incorporate a current in plane design (CIP) in which current flows from one side of the sensor to the other in the plane of the layers described above. Another type of GMR sensor termed a current perpendicular to plane (CPP) sensor has a design in which current flows through the sensor in a direction perpendicular to the plane of the sensors.
A Magnetic Tunnel Junction sensor (MTJ) is comprised of two ferromagnetic layers separated by a thin insulating tunnel barrier layer and is based on the phenomenon of spin-polarized electron tunneling. The insulating tunnel barrier layer is thin enough that quantum mechanical tunneling occurs between the ferromagnetic layers. The tunneling phenomenon is electron-spin dependent, making the magnetic response of the MTJ a function of the relative orientations and spin polarizations of the two ferromagnetic layers.
Both GMR sensors and TMR sensors require a mechanism for maintaining the pinned layer in its pinned state. Traditionally, this has been achieved by depositing the pinned layer such that it is exchange coupled with an antiferromagnetic material such as for example PtMn. Although an antiferromagnetic material in and of itself is not magnetic, when exchange coupled with a magnetic material it very strongly fixes the magnetization of the magnetic layer, In order to effectively fix the magnetization of the pinned layer, the antiferromagnetic layer must be very thick as compared with the other layers of the sensor. Ever increasing recording density requirements require ever smaller gap height and therefore thinner sensors. The thick AFM layer is a significant cost to the thickness budget. Also, antiferromagnetic materials lose there antiferromagnetic properties at a given temperature called the blocking temperature. Therefore, certain events such as an electrostatic discharge or a slider contacting the disk can elevate the temperature of the AFM sufficiently to lose the pinning of the pinned layer. Such an event renders the head useless.
In order to further improve pinning of the pinned layer, heads have recently been constructed with anti-parallel coupled pinned layers (AP coupled pinned layers). In such a sensor the pinned layer consists of a pair of ferromagnetic layers separated by a non-magnetic coupling layer such as Ru. The ferromagnetic layers are usually constructed to have magnetic thicknesses that are close to each other but not exactly the same. The antiparallel magnetostatic coupling of the two ferromagnetic layers greatly increases the pinning, and the slight difference in magnetic thicknesses creates a net magnetism that allows magnetic orientation of the AP coupled pinned layer to be set in a magnetic field. In such an AP coupled pinned layer, the ferromagnetic layer furthest from the sensor's spacer layer is exchange coupled with an AFM as discussed in the preceding paragraph.
Even more recently, in order to minimize sensor height and thereby increase data density, attempts have been made to construct sensors in which the pinned layer does not require exchange coupling to an AFM. Although some such sensors have used a thin seed layer of PtMn to initiate the desired crystallographic structure in the pinned layer, the thickness of the PtMn in such self pinned sensors is much too thin to exchange couple and pin the pinned layer. Such self pinned sensors achieve pinning through a combination of intrinsic anisotropy of the pinned layer, magnetostatic coupling, and magnetostriction. The two antiparallel coupled ferromagnetic layers are constructed to have as close as possible a magnetic thickness. The closer the magnetic thickness, the stronger the magnetostatic coupling between the layers. The antiparallel coupled layers are also constructed of a material having a strong positive magnetostriction. Magnetostriction is the property of a material that it is magnetized in a particular direction when placed under a compressive stress. The construction of the head generates a certain amount of compressive stress on the sensor which, when combined with the magnetostriction of the pinned layers, assists pinning. Such self pinned sensors have shown promise in greatly decreasing the thickness of the sensor, however they suffer from instability. The pinned layers of such sensor have been prone to flip direction, a catastrophic event which renders the head useless. Therefore, there remains a need for a mechanism for decreasing the thickness budget by using a self pinned layer while providing improved stability to the pinned layer to avoid amplitude flipping.